Mystery Migration of the Tiny Silvereye

A feat of remarkable endurance is currently underway. Tiny birds no more than 13 centimetres long are making an annual 1,600 kilometre trek from Tasmania, across Bass Strait, and as far north as Queensland. It's the equivalent of a human swimming from Auckland to Sydney, then walking to Cairo.

The Silvereye (Zosterops lateralis) is a small bird that weighs about 10 grams. Its name came about because of the thin white rings of colouring around its eyes that give the impression it is wearing silver-rimmed glasses.

They make their homes in a range of vegetation including eucalypt forests and woodlands, mallee, heath and mangroves.

While Silvereyes are native to Australia, they are also native to a number of other countries including Africa, Asia and the South Pacific region as far east as Fiji.

The Silvereye is an adaptable bird capable of spreading to new habitats with ease. In the 1830s they turned up in New Zealand, apparently after being blown off course when crossing Bass Strait from Tasmania, and they are now common in that country.

There are several races of Silvereyes, but it's only the Tasmanian race that performs the incredible migration across Bass Strait and up the east coast of Australia. Other races do migrate and cover considerable distances, but they don't compete when it comes to this annual long-distance migration.

Mystery migration

It's usually April before the Silvereyes start to migrate north and, by May, many of these amazing birds are turning up along the coast of southern and central New South Wales and some as far north as Queensland.

But this migration is a complex and mysterious business. Some Tasmanian Silvereyes don't migrate at all, preferring instead to rug up and spend winter on the Apple Isle. Others strike out across the water and head west when they hit the mainland, ending up as far as Adelaide.

Crossing Bass Strait is the biggest hurdle for the tiny Silvereye, and researchers are still working out how they do it.

Researchers believe it's most likely they achieve this feat by island-hopping, breaking the distance between Tasmanian and the mainland into several shorter stages.

This theory is based on the fact that large flocks of Silvereyes have been sighted on various islands in Bass Strait during both autumn and spring.

If they traverse Bass Strait via the arc of islands stretching from north-east Tasmania to Mornington Peninsular in Victoria, the longest non-stop flight over water is reduced to around 50km, which is still a long way when you're the size of a sparrow!

Once they hit the mainland, they often hook up with their mainland cousins and form large mixed flocks for their summer holiday sojourn. Some flocks linger around the same small mainland region while others tend to migrate northward following defined routes along the coastal plains.

Most migration takes place at night, or during dawn and dusk, but the flocks tend to drift in their migratory direction as they feed during the day.

Fact file:

When: April, May

Where: Tasmania to NSW, Queensland and South Australia

Other info: Migration is a complex and mysterious business. Some Tasmanian Silvereyes don't migrate at all, preferring instead to rug up and spend winter on the Apple Isle. Others strike out across the water and head west when they hit the mainland, ending up as far as Adelaide.

It takes a keen eye to pick the difference between the migrating Tasmanian Silvereye (left) and the mainland Silvereye (right)
(Source: Paul Willis)

Spot the difference

Given that the Tasmanian Silvereye mixes so readily with its mainland cousins, how can you know which type of Silvereye you are looking at?

Most of the time you'll find it difficult to tell the two apart! Silvereyes are small and evasive, as well as fast moving, which combine to make accurate identification quite difficult.

The Tasmanian Silvereyes are a little larger and heavier than the mainland variety, but not so much so that you'd notice at a fleeting glance.

There are, however, also some telltale differences in the plumage. The Tasmanian Silvereye is generally a darker bird and has dark reddish-brown coloured flanks. Its throat is whitish or greyish in colour while the non-migrants have a yellow throat and much lighter orange-brown flanks.

If you are sharp-eyed and can manage to get a good view for long enough, your best chance of picking the differences is when the Tasmanian and mainland Silvereyes are sitting side-by-side.

Tracking their progress

The movements of migrating birds like Silvereyes are detected and recorded through bird banding programs in which birds are caught, tagged with an individual number on a metallic band around their leg, and released.

Mist nets are used to catch the tiny Silvereyes at the Mt Annan Botanic Gardens
(Source: Paul Willis)

This way individual birds can be tracked and their movements monitored.

One pair of Tasmanian Silvereyes was caught in the same Sydney garden three times over a four-year period.

This indicates that the birds follow very exact migration paths year after year.

Other banded birds have been recaptured hundreds of kilometres from where they were first caught.

One Silvereye, first banded near Murwillumbah on the far north coast of New South Wales, was later recaptured near the small Tasmanian town of Don near Devonport, around 1,600 kilometres away.

Bird banding has also revealed that the average age of a Silvereye is about two years, although the oldest so far recorded has seen at least 11 birthdays.

How do they do it?

Very little is known about how Silvereyes manage to complete their epic migration every year. It's unclear how they know where to go, what makes them want to go and how they fuel the trip.

These questions have formed the basis of a research project by Dr Ursula Munro at the University of Technology in Sydney. Dr Munro has already made some inroads into understanding how the Silvereye knows where to go.

By placing them in artificial magnetic fields, Dr Munro has discovered that these tiny birds can read the magnetic field of the earth. But their migratory feat is more complicated than that.

Other experiments have shown they also use polarised light and can take their bearings from the setting and rising sun.

There is also a portion of their navigation system that is learned and involves the recording of key landmarks along their route.

Why do they do it?

The reason why they make this long and treacherous journey in the first place is another question entirely. Why risk travelling back across Bass Strait in spring when there's plenty of food available on the mainland?

If Dr Munro is right, it's all in their genes. She thinks that the travel bug is hard-wired into their tiny brains.

Dr Munro explained that it's a bit like having a little computer program that tells them when to leave, where to go, how long they need to travel, when to stop, and when to return.

Food resources also control the urge - if food is low, then it's time to leave Tasmania and head north.

About 10,000 years ago there was no water covering Bass Strait and Tasmania was connected to the rest of Australia by land.

The annual migration up and down the coast would have been much less hazardous for the little Silvereye with dry land stretching all the way from Hobart to Brisbane and beyond. Under these conditions, migrating made a lot of sense and allowed them to make full use of seasonal gluts of food in the different regions.

When the last Ice Age finished, sea levels rose and Bass Strait was formed, but the Silvereye's urge to migrate was so strong it forced them to maintain their yearly pilgrimage across a perilous 200km of water.

Another question that remains to be answered is where a small 10 gram bird like the Silvereye finds the energy for such a long flight.

Dr Munro believes they probably build up a deposit of fat during the summer that is used to fuel their long autumn flights.

To confirm this theory, Dr Munro is currently comparing the feeding habits of the Tasmanian Silvereye with the mainland Silvereye to discover any differences that might explain how the migrating Tasmanian Silvereye gets its fuel.

Although it is still early days, she is already discovering differences in their feeding patterns that may answer this question.